The invention relates to a multiaxis joint, especially an artificial knee joint, whose axes of rotation are perpendicular to one plane.
In prosthetics, artificial knee joints are known which swivel about a single axis, their respective swiveling movement being unsatisfactory, and which in particular permit only an unnatural gait pattern. Multiaxis knee joints have therefore been disclosed whose axes are perpendicular to one plane, for example in accordance with U.S. Pat. No. 5,314,498.
In multiaxis knee joints of this type, a polar curve describes the relative movement of the instantaneous center of rotation during swiveling. In this way, a harmonious gait pattern is obtained, because walking on uneven surfaces and stepping over small obstacles and the like is made easier by a shortening of the knee joint. To do this, the leg has to be consciously lifted and, in the process, the instantaneous center of rotation generally shifts forward before the joint comes into the actual flexion angle. In this forwardly directed movement, the points of articulation move away from one another, which results in a lengthening of the knee joint. One reason for this, among others, is that the polar curve and thus the instantaneous center of rotation generally lies outside the actual knee joint. In this respect, an unnatural pattern of movement is also produced here, because the leg which is actually to be shortened is first lengthened and must therefore be lifted higher than an obstacle or the normal pattern of movement actually requires.
Against this technical background, the invention has the object of making available a multiaxis joint, especially an artificial knee joint, which permits a harmonious gait modeled after the natural function of a knee.
To achieve this object in a multiaxis joint, especially an artificial knee joint, whose axes of rotation are perpendicular to one plane, a relative swiveling of an upper platform toward a lower platform from a rest position to a position of maximum excursion is permitted, upon which swiveling the distance between the platforms is immediately reduced with an excursion from the rest position. The polar curve which describes the relative movement of the instantaneous center of rotation will as a result also immediately describe a convergence of the two platforms which are attached to a thigh and lower leg, respectively. Thus, upon a walking movement and a lifting of the prosthesis, the distance between a ground surface and a foot of the prosthesis is immediately increased to such an extent that irregularities and small obstacles can be surmounted in an almost natural way.
In particular, it is also provided for here that, during the swiveling of the platforms relative to one another, the instantaneous center of rotation at all times lies inside the actual joint.
Here, the platforms are each understood to be an attachment element which serves to bind the joint according to the invention to a femoral stump and to a below-knee prosthesis, respectively. In this respect, the spatial configuration of the platforms is open.
In terms of construction, provision can be made for the joint to be designed to be self-retaining or self-locking in the rest position and/or the position of maximum excursion, or for these positions to be marked by dead-point positions of the joint. This affords the advantage, in particular in the rest position, that the joint is as it were locked in the respective position even when loaded. This results, on the one hand, in a stable and unstressed stance and, on the other hand, buckling of the joint upon heel contact in the walking cycle is avoided. In this respect, a particular aim is to inhibit the joint in a range of excursion from one of the positions of between 0xc2x0 and 10xc2x0. Nevertheless, initiation of the flexion phase starting from the rest position, in particular also starting at 0xc2x0, is possible without any problem.
Alternatively, or in addition, provision can be made for the rest position and/or the position of maximum excursion to be fixed by one or more limit stops. Such limit stops guarantee that the swiveling can take place only within a predetermined angle range. By means of a limit stop, it is also possible for a considerably greater load to be taken up by the joint in said positions. It has proven expedient to provide for a cushioned run-in into the rest position and/or the position of maximum excursion. In this connection, it is conceivable on the one hand to provide special dampers, for example also on limit stops, or in some other way to reduce the angular velocity of the axes of rotation during the run-in into the positions. A cushioned, gentle run-in into one of the positions can also be achieved by the predetermined kinematics of the joint itself, for example by the run-in into self-retaining, self-locking areas or dead-point positions.
In terms of construction, provision is made for the platforms to be connected by two levers of different length articulated respectively at their ends on the platforms.
This measure ensures that the upper platform is displaced variably not only in height relative to the lower, but that the platforms are actually swiveled about an angle relative to one another. As a further construction measure, provision can be made that in one position, in particular in the rest position, the opening angle of the lines connecting the axes of the levers is less than 95xc2x0, and in particular these can also be approximately perpendicular to one another. In this respect, it is preferable that, in this position in particular, an axis of the second lever is arranged lying substantially on the connecting line of the axes of a first lever. A setting of the joint is thus obtained such that in normal standing, when the joint assumes the rest position, a stable and unstressed stance is guaranteed. This up to and including a certain excursion, if loading takes place in this excursion.
In this respect, it is also preferably provided that, during the swiveling, the levers cross each other, or at least in continuation the connecting lines of the first and second axes. These measures ensure, inter alia, that the instantaneous center of rotation during swiveling lies inside the joint according to the invention and that, as a consequence, an approximately natural pattern of movement is obtained.
In a further embodiment, it is provided that in one position, in particular in the rest position, the first axes of the levers articulated on the lower platform are arranged substantially at the same height or, alternatively, that a first axis of a lever articulated on the lower platform is arranged lying higher than the first axis of the other lever. Opposite the plane spanned by these two first axes in one position, in particular in the rest position, an angle of between 45xc2x0 and 75xc2x0, in particular of between 50xc2x0 and 60xc2x0, is preferably spanned between the first lever and this plane. This is the angle between the line connecting the axes of the first lever and the line connecting the first axes in the lower platform.
These construction measures are geared toward the natural attachment of the cruciate ligaments to the lower leg, and the natural articulation of the anterior cruciate ligament on the lower leg in the standing position. Corresponding to the distance between the attachment of anterior and posterior cruciate ligament to the lower leg, it is also preferably provided that the distance between the first axes is between 36 mm and 62 mm. Likewise, the distance of the first axis from the second axis of the first lever articulated on the upper platform can be between 45 mm and 80 mm, once again geared toward the length of an anterior cruciate ligament between thigh and lower leg of a person of average height. Corresponding to the distance between the attachment of the cruciate ligaments to the thigh, a distance between the second axes of between 16 mm and 46 mm is provided. The simulation of the human knee in respect of the cruciate ligaments is completed by the fact that, corresponding to the length of the posterior cruciate ligament, the distance of the first axis from the second axis of the second lever articulated on the upper platform is between 27 mm and 62 mm.
As a result of these construction measures, the situation is also achieved that in the rest position, i.e. in the standing position, the line of the principally vertical loading of the joint extends lying preferably between the two upper axes, and possibly also slightly in front of the second axis of the second lever, viewed in the direction of swiveling, but which line is actually crossed by the levers.
Corresponding to the natural movement, a restriction on the excursion of the joint is also provided by means of corresponding dead-point positions or limit stops at between 130xc2x0 and 175xc2x0.
To ensure a sufficient stability of the knee joint according to the invention, in particular in the rest position, it is possible, as a further construction measure, to provide for one lever to be designed as a double lever. This can be the first or the second lever, or, if appropriate, both levers can be designed as double levers at the end of a common shaft for example. Such a joint then satisfies the most stringent loading requirements.
In a preferred embodiment, the joint according to the invention is provided with a counterrecoil mechanism which, when the loading of the joint is released, for example when the leg is lifted, initiates a naturally acting swivel movement. Such a counterrecoil mechanism is generally arranged between upper and lower platform, so that any forces arising only impact there.
It is expedient, in the rest position, to bind the counterrecoil mechanism vertically between the axes of the upper platform. There is sufficient space available there for construction measures, and force introduction there is also favorable for excursion of the joint as a result of the lever arrangements.
In a construction embodiment, it is provided that at least one lever of the counterrecoil mechanism is articulated on a shoulder protruding from the upper platform. As a result of this measure, the lever, even upon swiveling of the joint, is free from the upper platform. The latter may also have recesses for the at least one lever of the counterrecoil mechanism, so that the excursion needed for the joint is guaranteed. To be able to take account of individual requirements, the distance of the axis of the articulation from the upper platform is expediently adjustable.
On the lower platform, the counterrecoil mechanism is preferably bound in a central seat via the guidance of an axially spring-mounted piston, which piston has the at least one lever articulated at its free end directed toward the second platform. It is further preferred for the piston to be guided on a normal axis, which extends substantially vertically in the rest position.
For at least one axis, a brake can further be provided for slowing down a rotational movement about this axis. In the joint according to the invention, the braking of a single axis is generally sufficient since the axes are connected to one another in the manner of a joint chain.
A brake is preferred, for which independent protection is also required, in which the braking force is generated by a weight loading the joint. In the rest position in particular, the joint is then as it were locked and ensures a very stable, comfortable stance, since in this stationary rest position the loading from the body weight is at its greatest.
In a construction embodiment, it is provided that the axis is guided in a mounting which has a longitudinal slit. In the manner of known belt brakes, braking is obtained by closure of the longitudinal slit and thus frictional gripping of the axis. This embodiment of a brake, in particular for the joint according to the invention, has the advantage that it takes up very little space.
It has proven expedient to allow the weight to act on the mounting of the axis via a lever arm, in particular via a lever arm whose active lever arm length is adjustable, that is to say in particular that the site of introduction of force into the lever arm is adjustable. It may also be conceivable actually to design the physical length of the lever arm so that it is variable,
for example by turning it in a thread. It is further preferred that the weight acts counter to the force of a spring. As a result of these measures, the response of the brake and the brake operation itself can be optimally adapted to the requirements of the prosthesis wearer.
In particular, it is further provided that in one position, in particular in the rest position, a brake force can be preset so as to take account of the individual requirements of a prosthesis wearer by making it possible to preset the force needed for excursion in particular from the rest position.
The joint according to the invention is first explained in greater detail on the basis of a functional model represented in the drawing, which functional model does not reveal the actual configuration as a knee joint in the prosthesis. Thereafter, a preferred illustrative embodiment of a joint, with counterrecoil mechanism and brake, shown in the rest position, is discussed in more detail. In the drawing: